docs: def _AddVariable(self, variable):
          """
          Add a variable to the model. Should not be used by end-user
          """
          if isinstance(variable, Signal):
              if not variable in self.signals:
                  self.signals.append(variable)
          elif isinstance(variable, Variable):
              if not variable in self.variables:
                  self.variables.append(variable)
          else:
              raise TypeError
          self._utd_graph = False